Protected branch cannot be updated through API with `block_branch_modification`

Why are we doing this work

Our fix for #435500 (closed) has introduced a regression that makes it impossible to use the Update a protected branch endpoint when a security policy applies that sets block_branch_modification.

Steps to reproduce

• Create a new project
• Create a protected branch
• Assign some security policy to the project that sets block_branch_modification
• Attempt to update an attribute that's not name with the Update a protected branch API and verify it results in 403 currently